The purpose of this work is to study the cells that participate in the effector arm of the immune response. The current emphasis is on chemotaxis, which is a mechanism by which cells can be attracted to inflammatory sites, delayed hypersensitivity reactions and growing tumors. The project includes histology of different types of delayed hypersensitivity lesion, chemistry of lymphocyte derived chemotactic factors and identification of substances that modulate chemotactic responses. BIBLIOGRAPHIC REFERENCES: Leonard, E.J. and Meltzer, M.S.: Characterization of mouse lymphocyte-derived chemotactic factor. Cell. Immunol. 26: 200-210, 1976. Meltzer, M.S., Stevenson, M.M. and Leonard, E.J.: Characterization of macrophage chemotaxis in tumor cell cultures and comparison with lymphocyte-derived chemotactic factors. Cancer Res. 37: 721-725, 1977.